Key management protocol is a key problem for Ad hoc network security. In this paper, we introduced a user contributed key generation strategy (UCKG) based on the identity based cryptography (IBC). Compared to IBC key generation strategy, UCKG has no PGC-cheating problem and no requirement of secure channel for key distribution. We also proposed a distributed key management protocol for UCKG and simulated it with NS-2. The simulation results show that while the network had the good connectivity, the distributed key management protocol has good performance.